This invention relates to a field emission cold cathode apparatus which has an emitter electrode and a gate electrode and which emits electron beams from emitters included in the emitter electrode. In particular, this invention relates to the field emission cold cathode apparatus comprising a heat radiating unit (or a heater) for decreasing adsorption of a gas into the emitters and, furthermore, to a display apparatus including the field emission cold cathode apparatus and a voltage supplying unit for driving the field emission cold cathode apparatus.
A field emission cold cathode apparatus of the type described comprises an emitter electrode and a gate electrode as main components and emits, by the tunnel effect, electron beams from a surface of emitters included in the emitter electrode without heating the emitters up. In the field emission cold cathode apparatus, a tunnel effect occurs in the emitter electrode, when the emitter electrode is given a predetermined negative potential relative to a potential of the gate electrode provided as a referential potential. This tunnel effect is caused to occur such that electrons are released out from the surface thereof. Thus, it is noted that electron-emission depends not on a temperature of each emitters but on a difference of voltages between the gate electrode and the emitters, namely, the emitter electrode.
Specifically, the emitter electrode has a substrate of a semiconductor or a conductor and a plurality of emitters arrayed on the substrate.
In general, electron beams emitted from the above apparatus mainly depend on the electric-field intensity at points of emitters. It is preferable that the electron beams have high current density by generating high field intensity. On the other hand, it is desirable that a voltage difference between the emitter electrode and the gate electrode is as small as possible. To this end, each of the emitters has a conical shape having a needle-like tip or apex.
The gate electrode is located in the vicinity of each emitter and has a plurality of holes, each of which surrounds the each emitter. Practically, a minimum distance between the tip of the emitter and the gate electrode is as small as one micron meter. With this structure, the gate electrode is supplied with a higher potential relative to the emitter electrode and serves to concentrate the electric field on the points of the emitters no as to extract or emit the electron beams from the emitters.
Furthermore, such a field emission cold cathode apparatus may comprise a focus electrode for focusing the electron beams extracted by the gate electrode and emitted from the emitters. The focus electrode is located above the gate electrode or around an outer periphery of the sate electrode.
In this structure, it is assumed that the focus electrode is supplied with a potential lower than the gate electrode. In this case, expansion of the electron beams extracted by the gate electrode can be avoided in dependency upon a difference of voltages between the focus electrode and the gate electrode.
This apparatus has a common problem that gas molecules are absorbed onto the surface of the emitters during emission of electrons. Gas molecules exit in a receptacle or chamber which accommodates this apparatus and which is kept at a low vacuum. In other cases when emitted electrons bombard components such as the gate electrode, an insulator surrounding the emitters an anode, and an inner wall of the receptacle, the gas molecules absorbed into the surfaces of the emitters are partially desorbed form in the form of ions. Such absorption of gas into the emitters undesirably increases a work function as regards electron-emission. In order to desirably continue emission of electrons from the emitters, the electric-field intensity must be maintained sufficiently high so that such absorption of gas into the emitters does not take place.
A conventional field emission cold cathode, which has an object solving the above problem, is disclosed in Japanese Unexamined Patent Publication (JP-A) Nos. 370635/92, 22038/92, 198255/93, 182969/95, 223705/96, 54639/89, and 310024/96.
JP-A No. 370635/92 provides a technique to decrease absorption of gas into emitter's surface in an image display device, by continuously or intermittently supplying emitters with a positive potential during no supply of the negative potential.
JP-A No. 22038/92 discloses a technique of dividing an emitter array into two groups of emitters. In this event, one group of emitters is kept at a potential higher than a gate electrode corresponding to the remaining one of groups during emission of electrons from the remaining one group, so that the electrons emitted from the remaining one group collide with the one group emitters and clean one group of emitters.
JP-A No. 198255/93 is an improvement of JP-A No. 22038/92 mentioned above and discloses a technique for supplying an anode with a negative potential during the cleaning of the one group of emitters. This technique is helpful to guide the electrons emitted from the remaining one group only onto the one group of the emitters.
All of the above mentioned conventional techniques, JP-A Nos. 370635/92, 22038/92, and 198255/93, make use of the electrodes basically included in the field emission cold cathode apparatus.
However, only making use of the basic electrode, prevention or restraint of absorbing gases can not be desirably accomplished.
The other conventional techniques, JP-A Nos. 182969/95, 223705/96, 54639/89, and 310024/96, disclose the way how to prevent or suppress absorption of a gas into surface of the emitters. Detailed contents of the techniques will be discussed below.
JP-A No. 182969/95 discloses a technique of heating a plurality of emitters with a heater formed under the emitter electrode having the emitters. This technique serves to effectively release molecule gases absorbed into the surface of emitters from the emitters.
JP-A No. 223705/96 provides a technique of causing a current to flow into a substrate. In this event, the substrate has a part which is located under the emitters and which is thin compared with other parts. With this structure, only the thin part of the substrate under the emitters is heated up when the current flows into the thin part. This structure also serves to desorb gases absorbed into the surface of the emitters.
JP-A No. 54239/89 proposes a method of releasing molecule gases absorbed into the surface of emitters from the emitters. In this method, the emitters are heated with a heater which is located on the same layer as the gate electrode and which surrounds a region of the emitters, namely, an emitter area.
JP-A No. 310024/96 suggests a technique of heating emitters up to release molecule gases absorbed into surfaces of emitters which are included in a part of a filament member. In this technique, a current is caused to flow between two electrodes.
However, the above-mentioned techniques or methods are disadvantageous in common in that the heater should be driven by a power supply which is independent of the power supply for driving the remaining components included in the apparatus.
Moreover, the technique provided in JP-A No. 223705/96 brings about complicated processes as a whole and a high cost, because a further process for deeply etching the back surface of the substrate is necessary in addition to the usual processes.
The technique proposed in JP-A No. 54239/89 has a problem that the heater surrounding the emitter area makes it difficult to form a member for attaining a focusing effect. In order to attain a good focusing effect, a focus electrode is often arranged to avoid expansion of electron beams emitted from the emitter and is located above the gate electrode or around an outer periphery of the gate electrode. With this structure, the focus electrode surrounds the emitter area. The focusing effect results from generation of an electric-field while the focus electrode is supplied with the negative potential. The emitted electron beams are guided by the focusing effect toward the anode. Therefore, it is preferable that the focus electrode is put in the vicinity of the emitter area, popularly, on the region surrounding the emitter area. However, the region surrounding the emitter area is occupied by the heater in JP-A 54239/89.
Consequently, the focus electrode and the like is restricted to a remote region except the region surrounding the emitter area. Thus. it is difficult for the structure of JP-A 54239/89 to attain a desirable focusing effect.
The technique suggested in JP-A No. 310024/96 has a complicated structure and cannot make use of the other conventional structures.
In addition, the other conventional techniques, JP-A Nos. 182969/95, 223705/96, 54639/89, and 310024/96, have a common drawback such that the number of terminals is inevitably increased to connect the heater to an independent terminal. Description about the above-mentioned drawbacks will be detailed.
A field emission cold cathode apparatus must have terminals connected to the electrodes and extended to the outside of the receptacle or housing which accommodates this apparatus. A field emission cold cathode apparatus acts in a vacuum, as atmosphere well-known manner. Actually using this apparatus, the receptacle is necessary to hold this apparatus in a vacuum. And, to supply the electrodes with potentials, the terminals should be extended to the outside of the receptacle.
As well known, spaces and locations for the terminals are limited by the structures relating to a size and the shape of the receptacle, such as a CRT and so on.
As understood above, to increase the number of terminals extended to the outside of the receptacle results in limitation of the space as mentioned above and shortens a distance between the terminals.
Practically, the limited space restricts the number of the extended terminals to an allowable degree, because a narrow distance between the terminals makes an electrical insulation characteristic worse.
However, no consideration is made at all in all of the other conventional techniques about the number of the terminals. In addition, no disclosure is also made about the problem which might take place when the number of the terminals increases.
Furthermore, costs of the apparatus undesirably becomes high as the number of the terminals increases. Herein, consideration is made about an electron tube which has a receptacle of ceramics. In this event, each terminals must be extended to the outside of the receptacle, which gives use to a lot of troubles. In addition, an increase of the terminals results in an increase of wires connected to the terminals. Such wiring process is time-consuming and costly because wire-bonding and welding are carried out. Just for reference, costs for the wiring process become very high as a single terminal is added when a TWT (Traveling-Wave Tube) is manufactured which is utilized in microwave range.